The imaging of an object through a tilted plane parallel plate results in an image exhibiting aberration. Each of spherical aberration, coma, and astigmatism are introduced into the resulting image. Such aberrations occur whenever the light rays are converging or diverging through the plate. Examples of previous attempts to solve such aberrations may be seen in U.S. Pat. Nos. 2,792,740 and 4,541,688. Also, see W. J. Smith, Modern Optical Engineering, McGraw Hill, New York, 1990, page 99.
A common prior technique for reducing the effects of aberrations from a tilted filter or optical plate is to reduce the thickness of the filter. However, this approach is effective only up to a point. Since the filter is in the imaging path, it must have a good surface figure, or planarity. As the filter is made thinner, it becomes increasingly difficult to make it flat. Specifically, the filter is made as thin as possible, while still maintaining the surface figure or planarity. The remaining filter plate aberrations are then left uncorrected so as to provide a problematical solution as required resolution becomes better.
However, in the construction of high definition television systems, small LCD panels lead to compact systems, and pixel sizes become very small resulting in a demand for improved optical performance. Pixel sizes become of the order of 51 by 36 .mu.m, but recombination filters can only be thinned to about 2.3 mm. Thinner filters typically have an unacceptable surface figure or flatness. A 2.3 mm thick filter introduces 52 .mu.m of astigmatism, but this amount of astigmatism seriously degrades the resolution of a displayed image.
Another prior solution has been to collimate the imaging cone passed through the filter. This makes, the rays parallel to the axis at the filter. However, this technique requires additional imaging optics and substantially increases the optical path, thereby resulting in a more costly and more complex system.
It may be possible to account for filter aberrations by design of the imaging optics, but this option can result in a significant cost increase for lens design and manufacture. Furthermore, it is not appropriate for systems where one channel exhibits an aberration and another doesn't. For example, in a color LCD projector the green channel does not experience such aberrations since the green channel light is not passed through any filters or plates.